The path is generally something like ~/My Documents/Paradox Interactive/Crusader Kings II/, where ~ is the home directory.
It will take you to a path that ends with something like steamapps\common\Crusader Kings II. Choose the local files tab and then press browse local files. It can be found via Steam: find CK2 in your games library, right click on the name and select properties in the menu. There are 2 different directories involved:
Install a good text editor (such as Notepad++), since default Notepad on Windows can possibly mess up the file encoding.
